1. Field of the Invention
This invention relates to a method for producing formed bodies. More particularly, the invention relates to a method for producing formed bodies from carbonaceous starting materials for raising the carbon content of metals.
2. Description of the Invention Background
The manufacture of metals may involve the deliberate addition of various chemical elements, or "addition elements," to the molten metal to alter the composition or properties of the metal in some desirable way. The desirable results achieved by using addition agents may include deoxidation of the molten metal to some desired degree, control of the grain size of the metal, improvement of the mechanical and physical properties and corrosion resistance of the finished metal, and increases in response of the metal to subsequent heat treatments.
One addition element, carbon, may be added to molten ferrous metals, for example steel or iron, to increase the carbon content of the metals and thereby increase the metals' hardness, tensile strength, and yield strength properties. In order to maximize the effect of the carbon-containing addition element and to avoid contamination of the molten metal with substances which may impart undesirable characteristics to the finished metal, it is desirable that the carbon-containing addition element, or "carbon raiser," be high in available carbon content and contain a minimum amount of non-carbon contaminants. Such undesirable contaminants may include, for example, sulphur, hydrogen, and nitrogen.
Known carbon raiser products have included coarse granules of graphite, particles of metallurgical coke or coal, petroleum derivatives or the same or similar carbonaceous substances processed in, for example, the form of briquets or pellets. While some of these carbon sources are inexpensive to use, they introduce impurities into the metal. Metallurgical coke, for example, contains excessive amounts of sulfur and nitrogen impurities. It is of paramount importance to minimize the impurities introduced into the metal.
Carbon raisers are emptied directly into the furnace during the production of the metal. To avoid problems the carbon raiser bodies must not be too large or too small. If the preformed carbon raiser bodies are too large, they may not dissolve quickly enough in the molten metal. The ratio of surface area to volume is too low. The undissolved carbon raiser product is drawn off in the slag and never gets into the metal. The extreme heat qenerated by the molten metal within the furnace causes heated gases to rise up the furnace flue, creating a strong updraft. If the preformed carbon raiser bodies have insufficient mass to oppose the upward gas current, they are driven up the flue and never reach the molten metal. In both situations, the result is lower than expected carbon content for the finished metal. More carbon raiser product is then necessary to achieve the desired carbon content in the metal. However, adding more of a carbon raiser product that contains impurities raises the impurities in the metal.
In order to form carbon raiser bodies of acceptable mass and size unitary bodies have been formed from agglomerations of the carbon raiser particles. It is, however, difficult to form such unitary bodies from the carbonaceous substances conventionally used as carbon raisers. Graphite is, for example, lubricous and very difficult to compress into briquettes or pellets. Coke fines have been shown to be difficult to briquette.
Known carbon raiser products utilize binders to bind particulate materials together into bodies having a sufficient mass and an acceptable ratio of surface area to mass. Examples include mineral-based substances such as sodium silicate, or carbohydrate-based substances such as dextrin, starch, or a molasses/sulfur mixture. For several reasons, carbon raiser bodies formed using these known binder substances have proven to be deficient.
An additional problem is the lack of physical integrity exhibited by the known carbon raiser products. Currently available preformed carbon raiser bodies employ highly volatile binders which burn out in the intense heat of the furnace. With the binder gone, the bodies fragment. Carbohydrate-based binders, for example, will burn out at between 390.degree.-500.degree. C. Many of the fragmented particles created when the binder burns out will have insufficient mass to reach the molten metal through the heated updraft. Also, carbon raiser products encounter physical stresses during both shipping and storage. Carbon raiser product is customarily made in the form of preformed units, such as briquets or pellets, and sold in bags or other containers which may include fifty or more pounds of product. The bags of carbon raiser bodies are often handled roughly and stored or shipped in stacks on pallets. The carbon raiser bodies in the bags are thus subjected to physical stress. The poor structural integrity of known carbon raiser products allows the individual briquets, pellets, or other preformed bodies to crack or crumble into smaller pieces, or to form dust. Such physical degradation is undesirable because the pieces or dust thus formed are often of insufficient mass to successfully oppose the furnace updrafts discussed above.
Binder substances used in the known carbon raiser products typically contain all or a major portion of substances other than carbon, requiring the addition of a greater amount of the product to raise the carbon content of the metal and the introduction of contaminants into the metal. For example, sodium silicate binder includes no carbon, while carbohydrate-based binders, although including carbon in their chemical structure, include no elemental carbon and also contain significant amounts of non-carbon elements. Carbon raiser bodies formed using these known binders will greatly reduce the efficiency of the carbon pickup. In the case of mineral-based binders such as sodium silicate most of the binder will dissolve upon contact with the metal and will then enter into the slag layer. Some sodium silicate may, however, re-infiltrate the molten metal. In the case of carbohydrate-based binders, most of the binding agent will volatilize out before reaching the surface of the molten metal.
In addition to the other disadvantages associated with non-carbonaceous binders used in known carbon raiser products, several of the carbon raiser products using such binders have been shown to add a low percentage of their total available carbon content (as low as 50%) to the finished metal.
In addition, the production of carbon raisers with either mineral or carbohydrate-based binders requires a significant addition of water when the binder is combined with the carbonaceous material. This moisture must then be removed from the completed briquets, requiring additional equipment, one or more additional steps in the production process, and additional expense.
Some known binders, such as dextrin or other carbohydrate-based derivatives, are hygroscopic. Preformed carbon raiser bodies incorporating such hygroscopic binders may physically degrade upon exposure to moisture and must be protected during shipment and storage from rain and other high moisture environments. Even if the moisture used in production of the carbon raiser bodies is removed, the moisture content of preformed carbon raiser bodies having hygroscopic binders can build over time just by exposure to damp air. If these bodies are not used promptly or, if stored for a time, thoroughly dried before their addition to a furnace, a steam explosion can result.
Thus, it is apparent from the aforementioned disadvantages of commercially available carbon raiser products that a need exists for carbon raiser bodies having improved structural integrity, a higher available carbon content, a low level of impurities, a non-hygroscopic nature, and a size and mass which ensures placement of a high portion of total available carbon into the molten metal.